Effect of propeller size on motor performance - RC Groups
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Jul 14, 2012, 12:10 PM
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Effect of propeller size on motor performance

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This thread will be much easier to follow and understand if you have prints of the 3 attachments to hand for ease of reference.
An alternative [better?] title for this thread could be "Typical characteristics of e-flight motors"

In this thread I attempt to show how a typical motor behaves with different loads. That is, with different size props.

All values relate to full-throttle operation. Speed variations result from fitting different-size props.

The motor used as an example is a 39 gram outrunner with a Kv of 1501rpm per volt. The supply voltage is 10.5, in accord with the maker's recommended 3S.

The horizontal axis of the graphs is motor speed, expressed as a percentage of the no-load speed.

The shape of the power curves is aproximately correct for any motor at any voltage, but of course the numbers for power, rpm, and prop size, refer only to the example motor.

The shape of the efficiency curve is typical, though the peak value and its %NLS position can be slightly different for other motors.

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Column 6 [in the table]

No prop fitted [= no load].

Motor runs at 15,800rpm. This is the no-load speed, the highest speed at this voltage.

Speed = supply volts x Kv = 10.5v x 1501rpm/v = 15,800rpm = NLS = 100%NLS

Power input is 10w [just abve zero]....... power output is zero.

Column 5

Prop fitted, 6 x 4.

Speed has reduced to 86% of no-load speed.

Power IN is 72w............power OUT is 54w.

This operating point coincides with the highest motor efficiency.

Be aware that peak effy is not the best operating point for most models, as motor weight is high in relation to power.

If the battery is sized for long flight time [eg; 40 minutes], in that case peak effy is a good operating point. High motor weight is compensated by low battery weight.

Column 4

Prop fitted, 7 x 4.

Speed has reduced to 75% of the no-load speed.

Power IN is 113w...........power OUT is 79w.

This operating point [75%NLS] is usually a good choice for most models [applies where the battery is sized to give about ten minutes run time at full throttle]

You can extract more power from this motor, but this is not advisible because it will probably result in overheat. If you want more power, then stay with 75%NLS, and choose a larger motor.

Note the power loss of 34watts. This is the undesired but unavoidable power which goes to heat up the motor. For continuous operation at WOT, the motor cooling arrangements must be able to remove heat at this rate when the motor temperature is still below the allowable max.

At this operating point motor temperature will probably be close to the safe limit. If much of the run time is at reduced throttle there should be no problem. But continuous WOT might be unsafe. I am assuming normal unimpeded cooling airflow.

Note that the input power / motor weight is close to the value of 3w/gram which is reccommended as a basis for motor selection by several respected pundits.

The supply voltage used in this example is 3S, as recommended by the motor maker. This is, presumably, in the maker's opionion, the highest practicable voltage for our application taking account of prop size and motor heating. What they regard as "practicable" we can only guess at. There is no agreed standard [as far as I know], so some inconsistency is to be expected.

Column 3

Prop fitted, 9 x 6.

Speed has reduced to 50%NLS.

Power IN is 211w..........power OUT is 106w.

This operating point results in maximum motor output power. It gives the lightest motor for a given output, but it can lead to high battery weight.

Note that the power loss [which heats up the motor] has drastically increased relative to column 3 [now 105w, from 34w]. Overheat is almost guaranteed, unless power bursts are very brief and separated by long cooling periods.

Advertised "peak power" and "max current" are often near this operating point. I have never seen these terms properly defined.The numbers are usually advertising hype, best ignored.


Column 2

Grossly oversize prop.

Speed reduced to 25%NLS.

Power IN is 309w.........power OUT is 83w.

This column illustrates why power IN [as shown on a wattmeter] is not a reliable indicator of power OUT [the power that reaches the propeller].

Compare column 2 to column 4. Three times the power IN...........seven times the power loss...........but only same power OUT.

Column 1

Motor prevented from turning.

Power In is 407w...........power OUT is zero.

Sometimes a motor fails to start; it stutters and oscillates.

If full throttle is applied, we get the condition described here. Twelve times the heating power compared to normal. Fries in seconds.

If a motor fails to start, chop the throttle without delay. Any further start attempts should be at very low throttle.
Last edited by peterangus; Dec 03, 2013 at 06:39 AM.
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Jan 02, 2013, 06:01 AM
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Jan 02, 2013, 12:04 PM
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Some terms explained

Power IN

This is the power supplied to the motor by the battery.

It is the power value shown by your wattmeter.

In eflight circles, it is casually refered to as "watts", which is unfortunate. It is not possible to engage in meaningful debate when using such vague terminology.

You may notice that there are three parameters in the table which have watts as their units.

Power OUT

This is the power supplied to the propeller via the motor shaft.

Obviously, this is an important parameter, but it is rarely mentioned in discussions.

We manage to get by without making use of this parameter because [for any correctly set-up system] power OUT has a roughly-fixed relationship with power IN. It is usually between 65% and 75% of power IN.

Power LOSS

This is the power which unavoidably goes to waste, and causes the motor to heat-up.

When a motor is operating at [say] 70% efficiency, then 30% of the power IN is heating the motor.

There is a limit to the capability of the cooling system. This, in turn, puts a limit on the allowable power IN.

We should take more interest in this parameter. It is easy to evaluate.
Last edited by peterangus; Jan 05, 2013 at 11:21 AM.
Aug 21, 2013, 08:09 AM
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Vendors' power claims

The power parameter quoted in adverts is almost always power IN.

For this motor, a realistic usable value is 113 watts.

But note the huge variability, depending on the load. This tempts the vendor to quote an "optimistic" value.

Any value quoted for power IN is meaningless unless it is at a stated % NLS.

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